My 2018 Tracker arrived yesterday, and after having had a chance to test it I can confirm the following:
It does enumerate and connect to the accessory correctly.
It does not transmit data sent by the accessory through the USB input
It does not continue tracking after the USB cable is connected
My tests were performed using the same firmware that worked perfectly immediately beforehand using the previous version of the Vive Tracker. I suppose it's possible that HTC have arbitrarily changed the interface and using different settings would make it work, but that's unlikely.
I cannot understand why HTC would release a v2 product that lacks key functionality the v1 product has without even an idea of if or when said functionality will be supported. Who thought that was a good idea?